Axial flow fan assembly

ABSTRACT

An axial flow fan assembly in which two or more fans in series provide successive fan stages which are close together and in which the average pitch angle of the blades of one stage differs appreciable from that of the blades of a previous stage.

Unite tates atet [191 Shipes et a1.

[58] Field of Search ..415/198, 199,219 R, 207, 415/208, 209, 210, 129,130; 417/424; 138/39-40; 416/127 [56] References Cited UNITED STATESPATENTS 1,402,869 1/1922 Knutson 415/198 1,909,611 5/1933 Charavay415/198 1 Aug. 20, 1974 2,313,413 3/1943 Weske 415/198 2,314,572 3/19432,466,827 4/1949 Roth 138/39 2,681,178 6/1954 Mart 26l/D1G. 11 2,908,33510/1959 Petty 415/129 2,982,361 5/1961 Rosen 416/127 3,357,496 12/1967Peterson 415/130 FOREIGN PATENTS OR APPLICATIONS 357,368 9/1931 GreatBritain 415/198 411,839 7/1945 Italy 415/199 R 1,181,926 1/1959 France415/130 77,645 1/1949 Czechoslovakia 415/129 Primary Examiner-Henry F.Raduazo Attorney, Agent, or Firm-W. F. l-Iyer; Marvin B. Eickenroht 5 7ABSTRACT An axial flow fan assembly in which two or more fans in seriesprovide successive fan stages which are close together and in which theaverage pitch angle of the blades of one stage differs appreciable fromthat of the blades of a previous stage.

3 Claims, 3 Drawing Figures AXIAL rLow FAN ASSEMBLY This invention is acontinuation of my copending application, Ser. No. 209,923, filed Dec.20, 1971, and entitled Axial Flow Fan Assembly" now abandoned.

This invention relates to axial flow fan assemblies for use in aircoolers or other industrial environments. More particularly, it relatesto improvements in fan assemblies wherein two or more fans are mountedin series to provide successive stages within a single fan ring.

When used in air coolers, fan assemblies are mounted either above orbelow tube bundles for performing useful work by causing air to passthereacross. In these and other industrial environments, the fans may bequite large, ranging in diameter from six to thirty feet. The amount ofwork to be performed by a fan is approximately a functionn of the cubeof its tip speed. At the same time, the noise generated by a fan is afunction of its tip speed to the fifth power. Consequently, the problemof excessive noise is compounded as the work requirement on the fanassembly is increased. At the same time, of course, in response topublic demand, our governmental bodies have, in regulating noise levels,adopted more stringent standards.

The historical purpose for using series fans has been to permit theuseful application of more air moving power to a given piece ofequipment than could be applied with only a single stage fan. In othercases, series fans have been used with the fan blades at somewhat lesserspeeds so as to move the same quantity of air as a single stage fan, butwith less noise.

However, in prior series axial flow fan assemblies of the lattercategory, the successive fan stages are of identical construction andaxially spaced apart a considerable distance, in some cases as much asthree fan diameters, because it has been shown that when such stages arenot widely spaced apart, they do not perform as much useful work and areless efficient. This phenomenon may be explained by several theories,one of which is that a certain distance between stages is required topermit the kinetic energy imparted to the air molecules by the upstreamfan to be converted to a pressure increase, and thus useful work. Oneauthority, William C. Osborne, states, in his book entitled Fans, thatunless the stages are widely spaced, the rotational velocity of the airleaving the upstream stage will modify operation of the succeeding stageor stages.

Regardless of the theory which is reasonable for this construction, thelarge spacing between fan stages of these assemblies causes them to bequite expensive, and, in some cases, to interfere with available headroom. Thus, the long shaft on which the fans are mounted, the large sizeand/or number of bearings necessary to mount the shaft, and theconsiderable height of the fan ring all contribute substantially to alarge capital investment.

The primary object of this invention is to provide a series axial flowfan assembly which is considerably less expensive to construct, butwhich is capable of performing substantially the same useful work atsubstantially the same noise level, as the above-described series fanassemblies.

This and other objects are accomplished, in accordance with theillustrated embodiment of this invention, by a series fan assembly inwhich it has been made possible to mount the fan stages considerablycloser together than heretofore thought possible, and preferablysubstantially adjacent one another, by providing the blades of thedownstream fan with a greater average pitch angle or pitch than theblades of the upstream fan. Thus, we have discovered that a series fanassembly of this construction operates without substantial loss ofefficiency or work potential, as compared with the above-described priorseries fan assemblies. At the same time, since the fans may be rotatedat the same speed as the fans of prior assemblies, they have theadvantage of the same low noise level. Of course, due to this closespacing of the stages, the over-all assembly is less expensive to buildand consumes less space due to the shorter shaft, the smaller and/orlesser number of bearings, and the correspondingly shorter fan ring.

It is also known that the blades of an individual fan have an optimumaverage pitch for accomplishing useful work under given conditions. Thisaverage pitch will usually range from about 10 to about 20 with respectto the horizontal, and when it substantially exceeds 20, the fan isgenerally found to be less efficient with little or no increase in airflow. We have found, however, that in the novel series fan assembly ofthe present invention, the average pitch of the blades of the downstreamfan may exceed 20 to a considerable extent without appreciable loss ofefficiency in the over-all fan assembly. Thus, the average pitch of theblades of the downstream fan does not place a serious limitation on thatof the blades of the upstream fan i.e., it does not place a practicalupper limit on either the average pitch of the blades of the upstreamfan or the difference in average pitch of the blades of the two fans.

One theory by which the surprising results of the present invention maybe explained is that the air stream leaves the upstream fan in modes ofhigh and low velocity, and, since it has not traveled the distanceheretofore allowed in prior assemblies to permit its kinetic energy tobe fully converted to velocity pressure, it is at a velocity at which itmay be picked up by the blades of relatively large average pitch.Regardless of the theory, however, the results have been demonstratedfor various operating conditions.

Its also known that each fan imparts a certain amount of swirl to theair, so that with series fans rotating in two stages, the swirl at theoutlet of the downstream fan is the sum of the two. It has been proposedto increase the efficiency of prior series fan assemblies bystraightening out this swirl with fixed vanes between fans. Inaccordance with the present invention, its proposed to realize a similaror greater increase in efficiency without the use of fixed vanes, andthus eliminate their cost and headroom, by increasing the average pitchof the blades of the downstream fan. Furthermore, we have found thatoptimum results are obtained when the blades of the downstream fan areso arranged circumferentially with respect to those of the upstream fanas to avoid the most turbulent part of the air stream from the upstreamfan.

We have also found that although the average pitch of the blades of thefans for producing optimum results are best determined by trial anderror, which is in any case rather standard practice in the fanindustry, such pitches may at least be approximated in accordance withcertain design criteria, as set forth to follow. We have further arrivedat additional criteria which is useful in setting the blades of the twofans in optimum circumferential relationship.

In the drawings, wherein like reference characters are used throughoutto designate like parts:

FIG. 1 is an elevational view of an air cooler which has been brokenaway in part to show a tube bundle and a fan assembly constructed inaccordance with the present invention and supported above the bundle fordrawing air upwardly thereacross;

FIG. 2 is a top plan view of the fan assembly; and

FIG. 3 is a graph showing a curve illustrating the results of testscomparing work and the efficiency of a series fan assembly soconstructed.

With reference now to the above-described drawings, the air cooler shownin FIG. I, and designated in its entirety by reference character 10,includes a tube bundle 11 mounted on vertical columns 12 above thesurface 13, and a tandem fan assembly 14 mounted above the tube bundleby means of a transition 15. As shown by the broken away portion of FIG.1, the bundle 11 includes a plurality of heat exchange tubes 16extending laterally between headers (not shown) at opposite ends of thebundles for conducting a process fluid to be cooled across the airstream induced in an upward direction by means of the fan assembly. Sidewalls 17 extend along opposite sides of the tube bundle from one headerto the other so as to confine air flow to the bundle.

The fan assembly 14 includes a cylindrical fan ring having upstream anddownstream series fans 19 and 20, respectively, providing successivestages mounted for rotation coaxially thereof. More particularly, thefans are of such diameter as to cause the tips of their blades 19a and20a to move closely and concentrically within the fan ring. Also, and asshown in FIG. I,-the blades of the fans are pitched to cause air to moveupwardly through the fan ring, and thus upwardly across the tube bundlein response to rotation of the fans in clockwise direction (lookingdownwardly). It is in this sense i.e., direction of air movement thatthe lower fan 19 is called upstream and the upper fan 20 is calleddownstream.

Both fans are mounted on a shaft 21 which extends vertically andcoaxially of the fan ring. The lower end of the shaft is driven by amotor 22 mounted on a motor support 23 suspended from the tube bundle orother portion of the air cooler in any suitable manner. The motor drivesa belt within a belt guard 24 disposed about the lower end of the shaftfor rotating the fans at a desired speed. The shaft is mounted forrotation at its upper end by means of a bearing 21a supported in the fanring 18 by radial struts 2111.

Each fan includes a hub 25 fixed to shaft 21 and having a plurality ofblade sockets 26 extending radially in equally spaced apart relation.The inner ends of the blades are releasably secured in the hubs, such asshown in US. Pat. No. 2,908,335, dated Oct. 13, 1959, which enables theaverage pitch on each blade to be adjusted about its spanwise axis asdesired, depending on operating conditions.

As previously described, and as will be apparent from FIG. I, the hubs25 of the fans 19 and 20, and thus the planes of the inner sides of thefans themselves, are substantially adjacent one another, whereby theaxial distance between the fans is at substantially a minimum. In anyevent, as compared with prior series fan assemblies, wherein thesuccessive stages may be axially spaced apart three fan diameters ormore, the fans 19 and 20 shown in FIG. I are spaced apart only afraction of their diameters. In actual practice, we have successfullytested series fan assemblies constructed in accordance with the presentinvention wherein fans 14 feet in diameter were spaced apart from thecenter of one hub to the center of the other hub a distance of only 20inches.

Thus, as will be apparent from FIG. 1, the fan ring 14 need be of heightnot substantially greater than the vertical axial thickness of the fans19 and 20, whereby it consumes a minimum of head room. Also, the shaft21 need be substantially no longer than that required to extend betweenthe fans and the motor drive, plus the axial thickness of the blades.Still further, generally only one bearing 21a of relatively small sizeis required as support for the fans adjacent the entrance of the fanring, whereas a fan assembly having the fans spaced three or morediameters apart would obviously require two or more relatively largebearings.

As shown, each blade tapers inwardly in a radially outward direction,and has a cross-section which is of generally air foil shape. In somecases, the opposite surfaces of the blades may twist to some extent, sothat the pitch, or angle which the active or upper blade face forms witha horizontal plane perpendicular to the axis of the shaft, may vary tosome extent along the length of the blade, and it is in this sense thatthe term average pitch is used herein. However, as is well known in theart, this variance is generally relatively small and thus insignificantinsofar as design considerations are concerned.

As previously described, the average pitch of the blades of the upstreamfan 19 is less than the average pitch of the blades of the downstreamfan 20. Also, as best shown in FIG. 2, and for reasons previouslymentioned, the blades of the two fans are staggered or angularly spacedapart in a circumferential sense. The respective average pitches of thetwo fans will depend on that required to accomplish the optimum usefulwork in a particular installation, and, as previously noted, althoughfinal adjustment in this reqard is normally obtained by trial and error,we have arrived at certain design criteria by which these angles, aswell as the staggering of the blades of the fans, may at least beapproximated.

In accordance with well known criteria, the work down by the fanassembly 14 is a function of the pressure drop (in inches of water) ofthe air across the tube bundle i.e., the difference between its pressuremeasured on its lower side and its upper side in causing a predeterminedvolume of air to move across the bundle. This requires consideration ofseveral factors, including tip speed of the fan blades, size of theblades, pitch of the blades, spacing between the tubes of the bundle orfree area, etc., which will be explained to follow, is also observed inthe design of the fan assembly of the present invention, with suchadditional considerations as are required in view of the novelconstruction of this fan assembly.

Although the fan assembly 14 is illustrated as part of an air cooler, itwill be understood that it may instead be used in other environments inwhich air is to be moved in order to perform useful work, such as inventilation, other types of heat transfer, combustion processes, and thelike. Thus, no novelty is claimed in the construction of the describedair cooler itself, other than that which results from the novelconstruction of the fan assembly 14.

As previously noted, the curve on the graph of FIG. 3 illustrates theresults of tests comparing the work and efficiency of a series fan bladeassembly having the pitches of the blades of its fans adjusted inaccordance with the present invention with that of a fan blade assemblyhaving the pitches of its fan blades otherwise adjusted. Thus, thecoordinates plotted on the graph represent work (measured staticpressure raised to the 1.6 power) and efficiency of fans in which theaverage pitch (in degrees) of the blades of the downstream fan isindicated above that of the blades of the upstream fan. As will beobserved from the curve, within a certain range, the useful work as wellas the efficiency of the assembly adjusted to follow the teachings ofthe present invention were greater than that of the assembly otherwiseadjusted, including by interpolation an adjustment to provide the bladesof both fans with the same average pitch. Outside of this range, wherethe differential between the average pitches of the fans became quitelarge, the efficiency decreased without an increase in useful work,indicating, of course, one end of the range. Although not of precisedefinition, a range so obtained will assist a person skilled in this artin arriving at desired pressures and efficiency for a giveninstallation.

In this test, each fan was 14 feet in diameter, had six blades, and wasrotated at a tip speed of 5,630 feet per minute. The centers of the hubsfor the fans were spaced apart 20 inches.

As previously mentioned, one theory for the surprising results of thepresent invention is that with the fan stages close together, thekinetic energy imparted to the air by the first stage has not been fullyconverted to a pressure increase. The design criteria for use inapproximating the pitches of the blades of the stages assumes that fullconversion takes place at a distance of 1.5 diameters from the first orupstream stage, and that, when the fan stages are spaced a lesserdistance apart, as in the present invention, the extent of conversion isdirectly proportional to this distance divided by 1.5 fan diameters.This assumption is based on an analogy of the air flow encountered inthe use of the present invention to the known characteristics of fluidflow through nozzles and orifices.

This design criteria involves application of the following equations:

VPO [Vaa/ 1 m (1) VP, [Van/66.651 (pa/p Va, 66.65 VP, (3)

Va, 66.65 VP (SP/2) [W Z'A ti -arctan (sin do) 0zarctan Vaa (Sin a) wa1-arctan VMVaOpa (7) R 0.67 [AS/D] wherein:

As Axial spacing between successive fan stages, ft.

D Fan Diameter, ft. R Fraction of kinetic energy converted to a pressureincrease SP Static pressure differential imposed on the air flow streamseparate from the fan, inches of water Va Axial component of airvelocity leaving first stage and entering second stage, ft/sec Va Axialcomponent of air velocity leaving second stage, ft/sec Vaa Average airvelocity in the axial direction,

ft/sec Vb Blade Velocity at tip, ft/sec VP Velocity pressure into firstfan stage, inches of water VP Velocity pressure leaving first fan stage,inches of water 0 Pitch of blades of first fan stage, degrees 0 Pitch ofblades of second fan stage, degrees 0 Swirl angle (degrees) of airentering first fan stage, with respect to the axial direction a, Swirlangle (degrees) of air entering second fan stage, with respect to theaxial direction pstd Standard air density, 0.075 lbs/ft (sea level,

F) pa Actual air density, lbs/ft. To illustrate application of theseequations, assume that:

SP 0.542 inches of water Vb ft/sec Vaa 24.85 ft/sec As 1.33 ft D 14 ftpa pstd 0.075 lbs/ft and that the first stage must generate a velocitypressure equivalent to SP/2, or 0.5 (0.542) 0.271. With respect to thefirst stage:

VP =[24.85/66.65] X (0.075/0.075) =0.l39, and VP =0.139 +(0.542/2)0.410, so that VP 66.65 (0.410)" 42.67 ft/sec. Since no swirl has beenimparted to the air stream prior to reaching the first fan stage, 01 iszero, and:

0, arctan [42.67/110] 21.2".

With respect to the second stage:

R 0.67 (1.33)/l4 0.064, so that Va 66.5 [0.139 0.27 1 (0.936) 0.271 154.3

ft/sec., and

The previously mentioned design criteria for determining thecircumferential stagger or offset between blades of the two stagesinvolves the application of the further equations:

and

166.5 (0.410) [110 (42.67) 0.075 and 36O (As)Vb 36O (tan aQAs 2N 1rVa Dn-D From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinabove set forth,together with other advantages which are obvious and which are inherentto the apparatus and method.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth or shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

The invention having been described, what is claimed is:

l. A series axial flow fan assembly, comprising a fan ring having aninlet and outlet and a portion therebetween which is of constant innerdiameter, a shaft extending coaxially within said fan ring portion,first and second axial flow fans each having blades of equal outerdiameter and a hub carrying the blades and mounted on the shaft forrotation in a direction to cause air to move from said first fan to saidsecond fan in a direction from said inlet to said outlet, and means foradjusting the pitch of each blade about its spanwise axis, the blades ofthe second fan being adjusted to an average pitch greater than theaverage pitch of the blades of the first fan in order to accomplishoptimum useful work, the blades of the second fan beingcircumferentially offset with respect to the blades of the first fan,and means for rotating the fans at a desired speed, the axial spacingbetween said fans being sufficient to permit the blades of each to avoidinterference with the blades of the other, when the blades of the fansare so adjusted, but considerably less than the minimum axial spacingthat would be necessary for the fans to accomplish the same useful workwith their blades having the same average pitch and rotated at suchdesired speed.

2. An assembly of the character defined in claim 1, wherein said fanring portion is virtually free of obstructions between said second fanand said outlet.

3. An assembly of the character defined in claim 1, wherein the planesin which the inner sides of the blades of the fans rotate aresubstantially adjacent one another.

1. A series axial flow fan assembly, comprising a fan ring having aninlet and outlet and a portion therebetween which is of constant innerdiameter, a shaft extending coaxially within said fan ring portion,first and second axial flow fans each having blades of equal outerdiameter and a hub carrying the blades and mounted on the shaft forrotation in a direction to cause air to move from said first fan to saidsecond fan in a direction from said inlet to said outlet, and means foradjusting the pitch of each blade about its spanwise axis, the blades ofthe second fan being adjusted to an average pitch greater than theaverage pitch of the blades of the first fan in order to accomplishoptimum useful work, the blades of the second fan beingcircumferentially offset with respect to the blades of the first fan,and means for rotating the fans at a desired speed, the axial spacingbetween said fans being sufficient to permit the blades of each to avoidinterference with the blades of the other, when the blades of the fansare so adjusted, but considerably less than the minimum axial spacingthat would be necessary for the fans to accomplish the same useful workwith their blades having the same average pitch and rotated at suchdesired speed.
 2. An assembly of the character defined in claim 1,wherein said fan ring portion is virtually free of obstructions betweensaid second fan and said outlet.
 3. An assembly of the character definedin claim 1, wherein the planes in which the inner sides of the blades ofthe fans rotate are substantially adjacent one another.